1. Field of the Invention
The present invention relates to a method for preparing a hydrophilic porous polymeric material.
2. Description of the Prior Art
Recently, the micro-porous products formed by different processings of bio-absorbable polymeric materials (such as collagen and PLGA) are widely applied in medical treatment, such as drug delivery carriers, constructs for bone and cartilage regeneration, templates for three-dimensional cell cultures and bioreactor substrate material. Being obtained from the Extra Cellular Matrix (ECM) of animal soft tissue, the collagen is extremely compatible with organisms and thus has great potential in biomedical applications.
Conventional processes used to prepare the porous collagen are freeze drying processes and only a few are critical point drying processes or air drying processes. The above processes are based on the research of Yannas and Burke since 1980. Please refer to: Design of an artificial skin. I. Basic design principles, I. V. Yannas; John F. Burke, Journal of Biomedical Materials Research. Vol.14, 65-81 (1980); and Design of an artificial skin. Part III. Control of pore structure, N. Dagalakis, J. Flink, I. V. Yannas et al., Journal of Biomedical Materials Research Vol. 14, 511-528 (1980).
1. Freeze Drying Process
One of the most common processes in recent use to prepare porous materials is freeze drying process and its preparation theories are disclosed in U.S. Pat. Nos. 4,412,947, 4,522,753, 5,869,080 and 5,723,508. Natural polymeric materials are well dispersed in an aqueous solution or a solvent, and the mixture is frozen. After that, the water or solvent in the mixture is solidified as ice crystals. The ice crystal inside the frozen mixture is directly sublimed under a high vacuum condition so as to form a porous polymeric material.
The porous polymeric material formed by the freeze drying process, has a very wide range of pore size, which depends on the size of ice crystal. It is possible to obtain a porous polymeric material having a high porosity (about 90% by volume) with interconnecting pores. The pore size and pore structure are influenced by factors such as freezing temperature, the amount of water contained in the aqueous solution, pH value and uniformity of dispersion. In 1986, Doillon et al. illustrated that the lower the freezing temperature or the faster the rate of temperature decrease, the smaller the ice crystals. Thus, a porous polymeric material with a smaller pore size is also produced. That is, the size of the ice crystal is to a large degree controlled by different freezing temperatures or different rates of decreasing temperature. Please refer to: Collagen-based wound dressings: Control of the pore structure and morphology, C. J. Doillon et al., Journal of Biomedical Materials Research Vol. 20, 1219-1228 (1986). For example, the pore size of about 14 xcexcm is prepared at a temperature of xe2x88x9280xc2x0 C., the pore size of about 30 xcexcm is prepared at xe2x88x9255xc2x0 C., and the pore size of about 100 xcexcm is prepared at xe2x88x9230xc2x0 C.
The freeze drying process is the most common one to produce a porous polymeric material. The advantage of the freeze drying process is that the water or solvent contained in the frozen mixture will be sublimed from its frozen state so that the change of the pore structure influenced by the chemical and the physic change is slight. Moreover, different pore sizes of the material can be produced by controlling the size of the ice crystal. Other solvents need not be introduced during the preparation process.
The shape of ice crystal influences the pore shape of the porous material because the pore is created by sublimation of ice crystals. However, well controlling of ice crystal structure and uniformity of its size is not easy to be achieved. Moreover, the freeze drying is a phase separation process, the water or solvent inside the frozen mixture needs to be xe2x80x9csucked outxe2x80x9d by a freezing dryer. Thus, the above process to form a porous polymeric material is quite expensive and time-consuming. In addition, the size of the porous polymeric material formed by the freeze drying process is difficult to be programmed. Besides, the porous polymeric material absorbs water easily and becomes wet. The porous polymeric material needs to be reacted with a cross-linking agent for a cross-linking reaction so as to keep the stability of the structure. However, the structure of the porous polymeric material is often tumbledown and damaged if the cross-linking reaction is performed in the solution. Thus, the cross-linking agent used for the cross-linking reaction is usually in its gaseous form, such as the gaseous glutaraldehyde, to prevent the pore structure from damage. The reaction of the gaseous glutaraldehyde has to be monitored for its connectration and residual amount. The residual glutaraldehyde can be removed by blowing with air or nitrogen, and it is very difficult to remove completely. The glutaraldehyde remaining in the porous polymeric material causes the tissue to calcify or toxify the cells. The above-mentioned factors are, therefore, unfavorable for a mass production of a porous polymeric material formed by the freeze drying process.
2. Critical Point Drying (CPD) Process
The critical point drying process is similar to the freeze drying process, except that the water or solvent (such as alcohol) used in the freeze drying process is replaced by a liquid carbon dioxide. The porous polymeric material is formed while the carbon dioxide is directly sublimed by heating. However, the porous polymeric material formed by this process will shrink about 70% and the diameter of the pore is about 15 micrometers. Therefore, the critical point drying process is usually applied to prepare samples of scanning electron microscope (SEM) in a laboratory, and is not applied for a mass production.
3. Air Drying Process
The air drying process is to evaporate the water or solvent contained in the polymeric material at room temperature. Different pore structures are formed while different solvents are used. The pore size formed by air drying process is not easy to control the size distribution and morphology. Moreover, the air drying process is also time-consuming and cannot produce a sponge-like product.
The first object of the present invention is to provide a method for preparing hydrophilic porous polymeric material, comprising the following steps:
(a) mixing a hydrophilic polymeric material with a hydrophobic material;
(b) solvent sintering the surface of the hydrophilic polymeric material by adding water or an aqueous solution; and
(c) removing the hydrophobic material contained within the hydrophilic polymeric material with an organic solvent, which can dissolve the hydrophobic material, and solidifying the sintered hydrophilic polymeric material.
The second object of the present invention is to provide a method for rapidly mass producing the porous polymeric material. The pore size of the porous polymeric material and its porosity are controlled by the mixing ratio of the hydrophilic polymeric material and hydrophobic material and their particle sizes.
The third object of the present invention is to provide a method for preparing a hydrophilic porous polymeric material which further comprises a cross-linking step so as to improve its mechanical strength and the resistance to an acid and a base and to reduce the immunoreactions caused by implantation in humans.
The fourth object of the present invention is to provide a method for preparing a hydrophilic porous polymeric material, in which the pore and the composition can have a gradient distribution to meet various requirements for application.